RFR: 8256516: Simplify clearing References

[Per Liden]

On Wed, 18 Nov 2020 10:10:33 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Please review this simplification of jlr.Reference clearing by VM code.
>> 
>> The function java_lang_ref_Reference::set_referent_raw was being used to
>> clear the referent of Reference objects, and only for that purpose.  This
>> change replaces that function with java_lang_ref_Reference::clear_referent,
>> which is much more obvious in intent.  That change is then percolated up
>> through callers in the obvious way.
>> 
>> Testing:
>> mach5 tier1
>
> src/hotspot/share/gc/shenandoah/shenandoahReferenceProcessor.cpp line 319:
> 
>> 317:   } else {
>> 318:     // Clear referent
>> 319:     reference_clear_referent(reference);
> 
> Now I am looking at this code and wonder if we could just inline `reference_clear_referent` and `reference_set_next` both in Shenandoah and ZGC code. Probably something for a followup.

I'd prefer to keep the `reference_*` helper functions as is in ZGC.

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PR: https://git.openjdk.java.net/jdk/pull/1286